import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * Description:类似于斐波那契数列
 */
public class TestDemo {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n =scanner.nextInt();
        int sum = 0;
        sum = frog(n);
        System.out.println(sum);
    }

    public static int frog(int n){
        if(n == 1 || n == 2){
            return n;
        } else{
            return frog(n-1)+frog(n-2);
        }
    }
}
